1. Field of the Invention
The present invention generally relates to a battery pack, and more specifically, to a battery pack which includes a rechargeable battery and a protection circuit which, upon detection of an over-charge, over-discharge or over-current of the rechargeable battery, operates a switch on a wire provided between the rechargeable battery and a load circuit or a charging device to be in an off state.
2. Description of the Related Art
Recently, lithium ion batteries have been provided as rechargeable batteries for use in portable apparatuses such as digital cameras or the like. Since lithium ion batteries can be damaged by use under over-charge conditions or over-discharge conditions, lithium ion batteries have been provided for use in battery packs that include protection circuits for over-charge conditions or over-discharge conditions.
FIGS. 1 and 2 show block diagrams of battery packs of the related art. In FIG. 1, it is shown that a series circuit of a resistor R1 and a capacitor C1 is connected in parallel to a lithium battery 2. An anode of the lithium battery 2 is connected to an external terminal 3 of the battery pack 1, and a cathode of the lithium battery 2 is connected to an external terminal 4 via N-type MOS (Metal-Oxide Semiconductor) transistors M1 and M2, which are used for limiting a current.
Drains (electrodes) of the MOS transistors M1 and M2 are connected in common, a source (electrode) of the transistor MOS M1 is connected to the cathode of the lithium ion battery 2 and a source of the transistor MOS M2 is connected to the external terminal 4 of the battery pack 1. Also, body diodes (parasitic diodes) D1 and D2, respectively, are connected between the source and drain of the MOS transistors M1 and M2.
A protection IC (Integrated Circuit) 5 includes an over-charge detection circuit, an over-discharge detection circuit, and an over-current detection circuit. For operation of the protection IC 5, power Vdd is supplied from the anode of the lithium ion battery 2 to the protection IC 5 via the resistor R1, and power Vss is supplied from the cathode of the lithium ion battery 2 to the protection IC 5.
As for the protection IC 5, when the over-discharge detection circuit or the over-current detection circuit detects, respectively, an over-discharge state or an over-current state, the protection IC 5 sets an output of DOUT to be at a low level causing the MOS transistor M1 to be in a cut-off state. When the overcharge detection circuit detects an overcharge state, the protection IC 5 sets an output of COUT to be at a low level causing the MOS transistor M2 to be in a cut-off state.
In the example shown in FIG. 2, the battery pack 1 includes a thermistor R3. One of the terminals of the thermistor R3 is connected to a terminal 6 of the battery pack 1 and the other terminal is connected to the external terminal 4. When being charged, a predetermined voltage is applied to the terminal 6 of the battery pack 1 via a divided resistance with a charging device. The voltage of the terminal 6 is varied as the resistance of the thermistor R3 changes according to temperature of the battery pack 1. The charging device detects the voltage (indicating a temperature of the battery pack 1) of the terminal 6, and if the temperature (indicated by the voltage) of the battery pack 6 is greater than a predetermined value, the charging device performs operations to stop charging the battery pack 1.
Further, in Japanese Patent Application Publication No. 2004-152580, it is described that a series connection of a diode and a temperature control element (PTC element: positive temperature coefficient element) is made for a rechargeable battery and another diode is connected to the rechargeable battery in parallel to and in a direction reverse to the PTC element. This configuration is made to prevent the PTC element from operating under a normal discharge condition of the rechargeable battery even if the temperature of the rechargeable battery is high.
The related art example of FIG. 1 does not include a temperature protection function for the battery pack 1. On the other hand, the related art example of FIG. 2 shows a temperature protection function for the battery pack 1. In this case, since a predetermined voltage is applied to the battery pack 1 via the divided resistance from the charging device, there may be a problem in that the temperature of the battery pack 1 cannot be detected accurately when the predetermined voltage of the charging device is varied or when the divided resistance includes tolerance.